The overall objective of this project is to identify the specific metabolic defect in the disease cystinosis with the hope that this knowledge might lead to a logical and successful treatment for children with this disease. Seven approaches are planned: 1. We plan to characterize cystinotic variants in vitro. We will use somatic cell hybridization techniques to search for genetic complementation between cultured cells from patients whom we suspect to have different types of cystinosis. 2. We plan to study the efflux of cystine and other amino acids from isolated cystinotic and normal control lysosomes. 3. We have recently found that the intracellular cystine content of cystinotic fibroblasts is extremely dependent upon the pH of the culture medium. The intracellular free-cystine content of cystinotic cells increases 6-fold as the media pH is increased from 6.7 to 8.1. A better understanding of this phenomenon might provide important clues towards understanding the basic defect in cystinosis. 4. We have developed a new technique to isolate and identify small sulfhydryl compounds in physiologic fluids. We plan to see if there are small, naturally occurring sulfhydryl compounds in normal lysosomes and reduce the cystine to cysteine allowing its efflux from the lysosome. 5. We are beginning preliminary clinical trials of the drug cysteamine in children with cystinosis. 6. We plan to see if there is a defect in protein-disulfide reduction in cystinotic cells. 7. We would eventually like to produce an animal model for cystinosis. This might be possible by isolating mutagenized cystinotic embryonal carcinoma cells by a recently reported selection technique and injecting these cells into blastocysts of pseudopregnant female mice.